Buck converters are generally used to transform a relatively higher DC voltage into a relatively lower DC voltage. An example of a buck converter is shown in FIG. 1. The converter comprises a high-side switch Mp 1 and a low-side switch Mn 2 connected in series between a supply 30 and a ground 4. The control terminal 11 of the high-side switch 1 is controlled by means of a switch controller 14, and the control terminal 21 of the low side switch to is controlled by switch controller 24. The switches are closed in alternation such that the half-bridge node 5 therebetween is alternately connected to supply 30 and to ground 4. An output filter consisting of an inductor L and capacitor C filter the switching signals to create a DC voltage at the output 35; the DC output voltage may be used to drive a load Rload as shown.
In the case that a negative voltage with respect to ground is applied to the input of a buck converter and the input voltage is sufficiently negative to forward bias the body diodes of the switches 1 and 2, a negative current can be the result, as shown at 101. If switches 1 and 2 are low-ohmic, which is typically the case for high efficiency converters, the resulting current may be high and lead to damage to the converter. Such a situation could occur in a range of applications, such as, for instance and without limitation, for converting the output voltage of photovoltaic cells, strings or panels, or other applications in which a variable voltage source is associated with a storage device, or other load, having a relatively fixed or invariant input voltage.
Similarly, such a situation could occur where a power source with a relatively fixed or invariant output voltage is associated with a storage device, such as a lithium ion based battery, having a more variable input voltage.
Further, in situations in which the output is charged via a path (shown as Vaux in FIG. 1) other than the input, current can flow into the input, as shown at 102.
In order to overcome these problems, it is known to add a diode 6 in anti-series with the power switches, thus preventing reverse currents to the input either from the power stage or the output as shown in FIG. 2. The series diode however has a significant voltage drop in normal operation, when it is forward biased. This results in power being dissipated in the diode which can have a significant impact on the efficiency of the buck converter, especially when low-ohmic power switches are used.
Japanese Patent application Publication J2006-174612 discloses a buck converter in which, in place of the diode 6, a transistor is connected in anti-series with the high-side switch to prevent reverse currents. The transistor is controlled by the control block which also controls the timing of the switching of the high-side and low-side switches. By replacing the diode with a transistor, the efficiency losses associated with the forward bias drop in the diode may be to a large extent avoided.